The invention relates to a probe for analysing the exhaust gases of an internal combustion engine and more particularly to the mechanical structure of such a probe, with a view to ensuring at one and the same time its easy industrial production, its proper operation in use and its reliability in time.
Probes for analysing the exhaust gases of internal combustion engines have as principal function the measurement of the oxygen concentration in these gases, so as to react through a computer on the petrol ratio in the air, at the intake, so as to improve the efficiency of the engine and reduce the unburnt gases at the exhaust.
More generally, this type of gas analysis probe is used for measuring the concentration of a chemical kind in a mixture, but their principal application in the form contemplated is for checking the combustion in engines.
Very generally, gas analysis probes use, for measuring the oxygen concentration, a sensor formed by a concentration pile which delivers, between its output terminals, a voltage according to Nernst's law. The conditions of use, in a turbulent and hot gas flow, compel the use of only solid components, and these concentration piles are often formed by a measuring electrode, made from platinum, which serves also as catalytic pot, a reference electrode made from nickel, and a material sensitive to oxygen such as a zirconium oxide ZrO.sub.2 or titanium oxide TiO.sub.2, the oxygen reference being provided by the air.
In practice, the concentration pile is in the form of a hollow ceramic body obtained by sintering zirconium oxide, having an outer face exposed to the exhaust gases and an inner face exposed to pure air. Platinum metalizations on the outer face and nickel metalizations on the inner face are deposited by any method conforming to the rules of the art, deposition by brush, by silk-screen printing or chemically. The production of a probe in this form presents a certain number of difficulties and disadvantages which are more especially:
sealing difficulties at the mounting portion of the cone in the metal body which serves for fixing it to the engine, the seal being however obligatory so as to maintain a stable pure-air reference;
poisoning of the platinum film deposited on the outer surface of the cone, by the lead which is present in the petrol. It follows that such oxygen-measuring probes are in practice not usable outside the States in which petrol is sold without lead;
the dimensions of the ceramic cone result, during start-up of the engine, in overheating thereof which is transmitted relatively slowly through the ceramic body, which is relatively insulating, and there follows a thermal shock which may cause, either damage to the ceramic body, so to the concentration pile, or leaks which accordingly falsify the pure-air reference, this latter being then polluted by exhaust gases.